Developmental regulation and chromosomal decondensation of the 68C glue gene cluster in Drosophila melanogaster

Year: 1990

DOI: 10.7907/qm6d-4135

The larval salivary gland secretion genes (Sgs-3, Sgs-7 and Sgs-8) at chromosome position 68C in Drosophila melanogaster, are developmentally regulated and coordinately expressed. Each gene codes for a component of the mucoprotein glue which is synthesized in the third instar salivary glands. Expression of these three genes is associated with the 68C intermolt puff present in the polytene chromosomes of the salivary gland secretory cells. Expression occurs only in the salivary glands of third instar larvae, and requires the steroid hormone ecdysterone. We show that synthesis of the 68C glue gene RNAs is prevented in larvae carrying trans-acting mutations within the Broad-Complex (BR-C), while a puff persists at 68C in these mutant larvae. We use the l(1)su(f)[superscript ts67g] mutation (which has reduced ecdysterone levels and also prevents 68C glue gene expression) and a mutation in the BR-C (nprl[superscript 3]) to study the cis-acting elements responsible for interaction with trans-acting factors by analyzing the protein:DNA contacts which occur upstream of the Sgs-3 glue gene during active synthesis, using in vivo DMS-footprinting. The proximal promoter can direct tissue- and stage-specific expression and is shown to possess three protein-binding domains. Comparison of contacts at these three domains between expressing and non-expressing tissues (including salivary glands from (l)su(f)[superscript ts67g] and nprl[superscript 3] mutant larvae) identifies a single binding domain responsible for controlling developmental expression of Sgs-3.

We also analyze the cis-acting sequences required for the chromosomal puffing associated with 68C glue gene expression. Examination of various fragments of 68C DNA reintroduced into the Drosophila chromosomes by P-element transformation identifies a region of 152 basepairs between Sgs-7 and Sgs-8 which is necessary, but not sufficient, to promote puffing. Only when this region is accompanied by an adjacent promoter element is there a puff. The insertion sites containing these reintroduced fragments fail to puff in mutant larvae; therefore, formation of the 68C intermolt puff requires the products of the su(f) and BR-C loci, and the puff at 68C in mutant larvae is not the same puff as that associated with expression of Sgs-3, Sgs-7 and Sgs-8.